We studied 188 patients having aortic valve replacement for isolated (normal mitral valve function or only minimal mitral regurgitation) valvular aortic stenosis (with or without associated aortic regurgitation) to determine if certain variables allowed one to predict preoperatively whether the aortic valve was congenitally malformed (either unicuspid or bicuspid) or tricuspid (with the stenosis being acquired). All 188 patients were more than 40 years of age at the time of valve replacement, and all had coronary angiograms and determination of the peak systolic pressure gradient across the stenotic valve preoperatively. The structure of the operatively excised valve was classified by the same observer. The mean age was 61 plus/minus 9 years; 139 were men and 49, women. Eighty patients (43%) had at least 1 coronary artery narrowed greater than 50% in diameter. Forty-three (23%) patients underwent coronary artery bypass grafting at the same time of aortic valve replacement. The mean peak systolic gradient across the aortic valve was 71 plus/minus 27 mmHg. One- hundred twenty-three patients had a total cholesterol greater than 200 mg/dl and 48 patients had a body mass index more than 27 kg/square m. A logistic regression model was developed which found 4 factors (age, total cholesterol, coronary artery disease, and body mass index) to be predictive of aortic valve structure: 1) patients with coronary artery disease and a body mass index greater than 27 kg/square m had a very low probability (5- 20%) of having a congenitally malformed valve; 2) patients aged less than or equal to 65 years and a total cholesterol less than or equal to 200 mg/dl had a very high probability (85-91%) of having a congenitally malformed valve.